As a method for producing (R)-2-[[[3-methyl-4-(2,2,2-trifluoroethoxy)-2-pyridinyl]methyl]sulfinyl]-1H-benzimidazole [hereinafter sometimes to be referred to as (R)-lansoprazole] or (S)-2-[[[3-methyl-4-(2,2,2-trifluoroethoxy)-2-pyridinyl]methyl]sulfinyl]-1H-benzimidazole [hereinafter sometimes to be referred to as (S)-lansoprazole] having an antiulcer activity, for example, JP-A-11-508590 (WO 97/02261) describes a method for optically purifying a compound product adjusted to contain an enriched enantiomer and crystallization method by removing the solvent, which comprises treating a compound containing either (+)-enantiomer or (−)-enantiomer in a greater amount, namely, a compound enriched in one enantiomer, with a solvent, selectively precipitating a racemic compound from the solvent utilizing the crystallinity of the racemate, filtering off the precipitated racemic compound and removing the solvent to give a single enantiomer of the compound having an increased optical purity, which corresponds to lansoprazole and the like.
JP-A-10-504290 (WO 96/02535) describes a production method of an optically active sulfoxide compound, which comprises subjecting a thio compound to an oxidation reaction, and crystallization (Example 11) of omeprazole, which comprises concentrating an acetonitrile solution and the like.
Lansoprazole is now on the market worldwide as a pharmaceutical product having a superior antiulcer activity. The crystal of lansoprazole is a racemate and is superior in preservation stability.
A crystal of optically active (R)-lansoprazole and (S)-lansoprazole obtained according to the above-mentioned conventional method does not necessarily satisfy the preservation stability, with the undeniable possibility of decreased purity, increased amounts of analogous materials, coloring and the like during preservation.
Therefore, there is a demand for a production method of the crystal of (R)-lansoprazole or (S)-lansoprazole sufficiently superior in the preservation stability.